Anchoring element

ABSTRACT

A screw-shaped anchoring element for permanent anchorage of hearing aid devices or ear and eye prostheses in the skull bone. A main body including external threads has a length less than 5 mm and is operative to be inserted into a hole in a skull bone. A flange is operative to provide a stop for the anchoring element when the anchoring element is screwed down into the comparatively thin skull bone. The flange has a substantially planar bottom surface for resting against the skull bone when the anchoring element has reached a final screwed-down position. A tool engaging socket is arranged at one end of the anchoring element. An inner bore extends at least partially through the main body at the first end of the anchoring element and is operative to connect one of a hearing aid device, an eye prosthesis or an ear prosthesis to the anchoring element. A self-tapping second end of the anchoring element includes at least one cavity and a cutting edge. The at least one cavity has a total volume corresponding to at least half a volume of bone tissue material that could be cut off by the cutting edge when the anchoring element is screwed down into a hole in the skull bone with a diameter corresponding to an inner diameter of the screw thread.

FIELD OF THE INVENTION

The present invention relates to a screw-shaped anchoring element(fixture) for permanent anchorage of hearing aid devices or extraoralprostheses in the form of ear and orbital prostheses in the skull bone.The invention is specifically intended to be used in connection withhearing aid devices of the bone conduction type, i e hearing aid devicesby which the sound is transmitted via the skull bone directly to theinner ear of a person with impaired hearing. However, the invention isnot limited to this specific application, but can be used in connectionwith other types of hearing aid devices for anchorage in the skull boneand for ear or orbital prostheses which are also anchored in the skullbone.

BACKGROUND OF THE INVENTION

For persons who cannot benefit from traditional, air conduction hearingaids there are other types of sound transmitting hearing aids on themarket, i e bone anchored hearing aids which mechanically transmit thesound information to a persons inner ear via the skull bone by means ofa vibrator. The hearing aid device is connected to an anchoring elementin the form of an implanted titanium screw installed in the bone behindthe external ear and the sound is transmitted via the skull bone to thecochlea (inner ear), i e the hearing aid works irrespective of a diseasein the middle ear or not. The bone anchoring principle means that theskin is penetrated which makes the vibratory transmission veryefficient.

This type of hearing aid device has been a revolution for therehabilitation of patients with certain types of impaired hearing. It isvery convenient for the patient and almost invisible with normal hairstyles. It can easily be connected to the implanted titanium fixture bymeans of a bayonet coupling or a snap in coupling. One example of thistype of hearing aid device is described in U.S. Pat. No. 4,498,461 andit is also referred to the BAHA® bone anchored hearing aid marketed byEntific Medical Systems in Göteborg.

The fixtures which have been used so far for the bone anchored hearingaid devices of the type which have been mentioned here as well as forexisting ear or orbital prostheses, have been designed in such a waythat a screw tap is required to form an internal thread in the holedrilled in the skull bone. One example of such a fixture illustrated inU.S. Des. 294,295. This fixture has an external thread with smallcutting edges with only a minor scraping effect in the pre-tapped bonehole. It has also a flange which functions as a stop against the bonesurface when the fixture is screwed down into the skull bone. The flangeis also in this case provided with through holes for bone ingrowth orthe like.

It is also previously known to use so-called self-tapping fixtures forpermanent anchorage of dental prostheses, dental implants, see forinstance U.S. Pat. No. 5,064,425 and U.S. Pat. No. 5,269,685, whichfixtures can be installed without the use of any screw taps. However,these types of fixtures which are used in the jaw-bone cannot be usedfor anchorage in the skull bone, which bone is much thinner than thejaw-bone. The dental implants (fixtures) are too long and they have verydeep, longitudinal bone cavities for collecting and retaining all thecut-off bone chips material.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a screw and anchoringelement (fixture) adapted for anchorage in the comparatively thin skullbone and which fixture is self-tapping so that no screw tapping isrequired and the installation can be carried out in a more simple way.

The invention is mainly characterized by the following features: thepart of the anchoring element which is intended to be inserted in theskull bone is shorter than 5 mm, the apical portion of the anchoringelement is self-tapping with at least one cavity with a cutting edge andthe totale volume of the cavity or cavities corresponds to at least halfof the cut-off bone tissue material when the anchoring element isscrewed down into a hole in the skull bone with a diameter correspondingto the inner diameter of the screw thread.

According to a preferred embodiment the cavities have a total volumewhich corresponds to 50–100% of the cut-off bone volume.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described more in detail inconnection with the accompanying drawings, in which

FIG. 1 is a side view of a self-tapping anchoring element according tothe invention,

FIG. 2 is an end view of the apical portion of the anchoring element,and

FIG. 3 is a side view of a self-tapping anchoring element according toanother embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a screw-shaped anchoring element, a so-called fixture1, according to the invention. The fixture is made of titanium which hasa known ability to integrate into surrounding bone tissue, so-calledosseointegration. The fixture has a main body 8 which is intended to beinstalled into the skull bone, a flange 7 which functions as a stopwhen-the fixture is installed into the skull bone and a tool engagingsocket 9 in the form of an external hex. The main body 8 is shorter than5 mm, in order not to go completely through the thin skull bone, and ithas a tapered apical portion 8′ and a straight, generally cylindricalportion 8″ with external threads. The screw thread diameter of the mainbody 8 is about 3.5–4.0 mm.

The apical part of the main body 8 is fitted with in this case threeself-tapping cutting edges 2 with a cutting angle 3 of 100° or lessformed by cavities 4 made in the main body material.

The cavities are designed in such a way that the total volume of thecavities 4 corresponds to at least half of the bone chips volume whichis cut-off when the anchoring element is screwed down into a boredrilled in the bone which bore has a diameter corresponding to the innerdiameter 5 of the screw thread. Preferably the cavities have a totalvolume which corresponds to 50–100% of the cut-off bone chips volume.

As illustrated in the figure the cavities 4 are not extending all theway along the cylindrical portion 8″ of the main body, they are onlyextending along the bottom or apical half of the cylindrical portion sothat a couple of full screw threads are remained above the cavitieswhich is important for the initial stability of the fixture. Thecavities are made only deep enough so that there is sufficient place forthe above-mentioned cut-off bone chips volume in the cavities.

That part of the screw body which follows behind the self-tappingcutting edge 2 when the screw is installed in the bone can be providedwith a clearance or relief surface 6. This design has two effects. Firstany squeezing effect between the screw and the bone during installationof the screw is reduced. And second, more volume for the cut-off bonechips is created.

The flange 7 has a planar bottom surface for resting against the outerbone surface when the screw has been screwed down into the small bone.The flange has a diameter which exceeds the peak diameter of the threadswith 10–20%. Extending between the flange 7 and the threaded part of themain body there is an unthreaded cylindrical part 10 having a diameterwhich corresponds to the inner diameter of the threads.

The fixture has an inner bore 11 with an internal screw thread forconnecting an hearing aid device or any orbital or ear prosthesis. Inorder to achieve a stable connection the inner bore is extending throughthe external hex 9 and all the way down into the bottom half of thecylindrical portion 8″ with the cavities.

The invention is not limited to the embodiment illustrated in thedrawing but can be varied within the scope of the accompanying claims.Specifically, it should be understood that the main body 8 which isintended to be inserted in the skull bone could be slightly taperedalong its length as shown in FIG. 3. Furthermore, the screw may bedesigned with an internal tool engaging socket instead of an externalone which has been illustrated in the drawing.

1. A screw-shaped anchoring element for permanent anchorage of hearingaid devices or ear and eye prostheses in the skull bone, the anchoringelement comprising: a main body comprising external threads, the mainbody having a length less than 5 mm and being operative to be insertedinto a hole in a skull bone; a flange operative to provide a stop forthe anchoring element when the anchoring element is screwed down intothe comparatively thin skull bone, the flange having a substantiallyplanar bottom surface for resting against the skull bone when theanchoring element has reached a final screwed-down position; a toolengaging socket arranged at one end of the anchoring element; aninternally threaded inner bore extending at least partially through themain body at the first end of the anchoring element and being operativeto connect one of a hearing aid device, an eye prosthesis or an earprosthesis to the anchoring element; and a self-tapping second end ofthe anchoring element, the self-tapping second end comprising at leastone cavity and a cutting edge, the at least one cavity having a totalvolume corresponding to at least half a volume of bone tissue materialthat could be cut off by the cutting edge when the anchoring element isscrewed down into a hole in the skull bone with a diameter correspondingto an inner diameter of the screw thread.
 2. The anchoring elementaccording to claim 1, wherein the at least one cavity has a total volumethat corresponds to 50–100% of the cut-off bone volume.
 3. The anchoringelement according to claim 1, wherein the flange has a diameter thatexceeds a peak diameter of the external screw thread of the main body byabout 10–20%.
 4. The anchoring element according to claim 1, wherein theself-tapping edges have a maximal cutting angle of about 100°.
 5. Theanchoring element according to claim 1, wherein the self-tapping edgescomprise a relief surface.
 6. The anchoring element according to claim1, wherein the main body further comprises an apical, tapered portion atthe second end, and wherein a portion of the main body including theexternal threads is straight and substantially cylindrical.
 7. Theanchoring element according to claim 1, wherein the main body furthercomprises an apical portion comprising a first tapering section, and aslightly tapered second tapering section extending from the flange tothe apical portion and provided with said external threads.
 8. Theanchoring element according to claim 1, wherein the main body furthercomprises a straight, cylindrical, unthreaded portion adjacent theplanar, bottom surface of the flange, the unthreaded portion having adiameter corresponding to the inner diameter of the screw threads. 9.The anchoring element according to claim 1, wherein the at least onecavity is arranged on a bottom half of the main body, wherein the bottomhalf is operative to be inserted into the skull bone.
 10. The anchoringelement according to claim 1, wherein the inner bore extends into abottom half of the main body where the cavities are arranged.
 11. Theanchoring element according to claim 1, wherein at least a portion ofthe threads extend entirely about the main body.
 12. The anchoringelement according to claim 1, wherein the at least one cavity has atotal volume substantially corresponding to the volume of bone tissuematerial.